Flexible printed circuit boards (or assemblies) have a wide variety of applications due to their low cost, flexibility and versatility. For instance, flexible printed circuit boards may be used in areas where space is limited, or where the surfaces upon which the printed circuits are mounted are not flat. Further, flexible printed circuit boards may be useful in dynamic applications, where the board is subjected to repeated flexing throughout its life, such as in disk drive heads, printer heads, and display board interconnects for portable computers. Also, when using suitable materials in manufacturing processes, flexible printed circuit boards may be constructed for use in extreme operating environments, such that they exhibit resistance to thermal and chemical exposure.
Typically, flexible printed circuit boards have been limited to single or double-sided constructions in order to provide sufficient flexibility and reliability. However, as packaging densities, processing speeds and information bandwidths have increased, a need has been created for higher density flexible printed circuit boards. One manner to increase the density of a flexible printed circuit board is to utilize three or more conductive layers sandwiched together, which is commonly referred to as a multilayer printed circuit board. By a "conductive layer", we mean a layer of conductive material which is typically arranged into a circuit pattern. A conductive layer may include elements such as signal, power, and ground traces, contact pads, heat sinks, active & passive electronic components, shielding patterns, alphanumeric designs (e.g. part numbers), etc. However, a number of characteristics related to the materials and manufacturing processes used to construct conventional multilayer circuits have made the conventional technology less than optimal for many flexible applications.
Conventional multilayer circuit boards have typically been constructed using adhesives to bond conductive layers to a dielectric substrate, as well as to join dielectric coverlayer films to the assemblies to insulate adjacent conductive layers. The adhesives are typically acrylics or epoxies. However, it has been found that the use of these adhesives in multilayer circuits significantly degrades their performance.
First, the use of adhesives produces relatively thick multilayer circuits. Increased thickness provides reduced flexibility, and may decrease the reliability of the multilayer circuit since many of the layers are located relatively far beyond the neutral axis of the multilayer circuit board, which increases the tension and compression forces to which these layers are subjected during flexing of the board.
The increased thickness of the multilayer circuits due to the use of adhesives also affects the thermal management capabilities of the board, since the ability of a circuit board to dissipate heat is a direct function of the thickness of the board. Decreased thermal management capabilities may result in lower performance and/or life of integrated circuit components which are active on the multilayer circuit board.
Increased thickness of multilayer circuits from the use of adhesives also results in the use of additional processing-steps and additional types and quantities of materials which must be used to construct multilayer circuits. Consequently, the cost of such multilayer circuits is increased.
The thickness of the layers on the multilayer circuit board due to the use of adhesives may also, in certain constructions, limit the packaging density of a multilayer circuit board. For instance, as discussed for example in U.S. patent application Ser. No. 08/001,811, which is assigned to the same assignee as the present invention, when using a conductive adhesive material to join interconnecting pads on two conductive layers through a dielectric coverlayer, it has been found that roughly a 25:1 ratio between the aperture size in the dielectric coverlayer and the connected distance between opposing pads must be utilized to ensure adequate connections with a conductive adhesive. By a "conductive adhesive", which is also often referred to as an "anisotropic adhesive", we mean an adhesive material which conducts through the thickness of the material (z-axis), while electrically insulating throughout the plane of the material (x-axis and y-axis). Since the adhesive material used to bond a dielectric coverlayer film to a substrate is relatively thick, the aperture size must conform to a 25:1 ratio in order to allow compression of the dielectric coverlayer during lamination of the conductive adhesive, which is required because conductive adhesives are not particularly well suited for bridging large distances. Consequently, by having a relatively thick dielectric coverlayer film, the aperture sizes of the electrical connections between conductive layers may waste a significant amount of space on the multilayer circuit board. For instance, using a 50 micron connected distance, the aperture size through this film must be at least 1250 microns. Compared to, for example, a conventional 125 micron conductive trace, it may be seen that the apertures may take up a significant amount of space on a multilayer circuit board.
The use of adhesives in multilayer printed circuit boards also limits the ability of the boards to withstand chemical and thermal extremes. Typically, the adhesives have a significantly greater susceptibility to chemical degradation than many dielectric and conductive materials. Chemical resistance is important in many applications, for instance, in some operating environments (e.g., brake fluid in ABS circuits) and in many common post-processing assembly steps. Also, the adhesives are typically less able to withstand high temperatures compared to the other materials in a multilayer printed circuit board. Further, it is often difficult to match the coefficient of thermal expansion (CTE) of the adhesive with the other materials in a multilayer printed circuit board. Consequently, during thermal cycling, the adhesive may expand at a different rate from the other materials, which may induce thermal stresses in the multilayer printed circuit board, possibly inducing failure of the board. Thus, the use of these adhesives is often a limiting factor on the performance of conventional multilayer printed circuit boards.
Another problem associated with many prior multilayer circuit assembly techniques is the use of high aspect ratio plated through holes to interconnect various conductive layers on a board. Conventional multilayer circuit construction techniques typically interconnect conductive layers by means of plated through holes which are formed throughout the entire thickness of a multilayer printed circuit board. Generally, all of the layers in a board are precisely aligned and bonded together, and then interconnecting holes are drilled, stamped, or otherwise formed at points of interconnection. The holes are then plated to form a conductive barrel structure.
Often, chemical or plasma etching is also required to etch away the substrates in the holes to expose portions of the conductive layers so that an adequate plating surface may be created in the through hole. This is a costly and time-consuming process, and it further increases the diameter of the through holes, wasting additional space on the board.
In multilayer circuits having a number of conductive layers, several problems exist with regard to these plated through holes. First, the holes are more difficult to plate as the aspect ratio (length v. hole diameter) increases, since it is more difficult to get material to adequately bond deeper inside the holes. As holes lengthen, their diameter must be increased to ensure adequate plating, which also wastes additional space on the board. Another problem with the plated through holes is that they may waste a significant amount of space on the multilayer printed circuit board, thus reducing the packaging density of the circuit, since in order to connect two layers in a multilayer circuit board, a hole must be formed through all of the layers in the board regardless of which layers are to be connected.
Another drawback to these high aspect ratio plated through holes is their susceptibility to breakage from z-axis expansion during thermal cycling. As the number of circuit layers increase in a multilayer circuit board, thermal cycling may induce failure in these holes since various materials such as adhesives in a conventional multilayer circuit board expand at different rates and induce thermal stresses in the z-axis of the circuit board. Consequently, it is desirable to limit the length of these holes in order to reduce the possibility of their failure from bending and/or thermal stress.
As a result of the limitations of conventional multilayer fabrication processes, the conventional wisdom has required a minimum of about 1 mil (or about 25.4 microns) of copper to be plated on the conductive layers and in the plated through holes to ensure adequate circuit board performance. MIL-P-50884 military specification for flex circuits, MIL-P-55110 military specification for rigid boards, and IPC 250 commercial specification for class II and class III boards (excluding low reliability, non-critical consumer applications in class I) all require a minimum of 1 mil thickness of copper in multilayer printed circuit boards. Class I devices allow as low as 0.5 mil (12.7 microns) thicknesses; however, low cost is the overriding concern in these applications, rather than reliability. Consequently, the Class I requirements are typically not sufficient for most applications where reliability is a significant concern.
Various adhesiveless laminate technologies have been developed in an attempt to alleviate the drawbacks associated with using adhesives. By "adhesiveless", we mean interconnects formed directly between two layers (an insulator and a conductor), without the inclusion of additional layers of conventionally recognized adhesive materials such as epoxies, acrylics, polyesters, cyanate esters, butyral phenolics, perflouropolymers, aramid perflouropolymers etc. Examples of adhesiveless technologies include application of a conductor to a base dielectric film, as in sputtering, chemical deposition, vacuum deposition, additive plating, or a combination thereof. Application of a dielectric to a base metal foil by casting and curing the liquid dielectric is also possible. However, it has been found that many adhesiveless processes are costly and time consuming, and not particularly suitable for high volume and/or low cost manufacturing. Further, many of these processes are not "dry" processes, and consequently, are not environmentally friendly. Casting processes also suffer from the drawback of having relatively low tear strengths, and it is also generally difficult to construct double-sided laminates by casting processes.
Attempts have also been made at eliminating the adhesive in bonding dielectric coverlayers or substrates in multilayer circuits. For instance, Volfson et al., U.S. Pat. No. 4,980,034; Kondo et al., U.S. Pat. No. 4,810,528; Dakos et al., U.S. Pat. No. 4,670,325; Davey et al., U.S. Pat. No. 3,622,384; European Patent Application 222,618; PCT Application 91/14015; and Japanese Application 4-176193, disclose various uses of screen-printed polyimide inks for use as dielectrics and/or substrates. However, many of these references disclose the use of such polyimide inks in rigid ceramic circuit boards, and many use an additive process to alternately print conductive and polyimide layers to form multilayer circuits. However, it has been generally found that the temperatures and pressures associated with such additive processes are beyond the capabilities of flexible multilayer printed circuit boards, in part due to the limitations of the adhesives and dielectrics used in these aforementioned conventional flexible multilayer circuits. For instance, conventional adhesives are generally unable to withstand the typical 270.degree. C. curing temperature for a dielectric ink. Gilleo et al., U.S. Pat. No. 4,747,211, discloses the use of a dielectric ink on a substrate utilizing polymer thick films to produce flexible multilayer circuits. However, this process is also unsuitable for use with conventional adhesive-based flexible multilayer circuit boards, again in part due to the inability of an adhesive to withstand the post-processing assembly steps associated with this process. European Patent Application 222,618 discloses a similar approach on ceramic substrates, but the 475.degree. C. firing step is unsuitable even for the best adhesiveless flex technologies because of the limitations of polyimide dielectrics.
Consequently, a need exists in the art for flexible multilayer printed circuit boards which generally do not suffer from many of the drawbacks of the conventional multilayer printed circuit boards. Among other needs, such properties as flexibility, reliability, high packaging density, and thermal and chemical resistance are desired. In part, it is desirable to reduce or eliminate the use of adhesives in bonding dielectric and conductive layers to a substrate in the formation of a multilayer circuit in order to reduce or eliminate the various limitations posed by the use of adhesives. It is also desirable to limit the overall thickness of the boards to further improve the above-described properties.